Method of producing resistors



I Sept. 1, 1936. I B. B. MINNIUM ETAL 2,052,839 7 METHOD OF PRODUCINGRESISTORS Filed Dec. 14, 1954 2, I 1 3 Q A 8 72 1 71 I3 ,5 1/5 3nnehtor@1701; E mafia/m BU Leifind EEK gr Patented Sept. 1, 1936 IVIETHOD FPRODUCING RESISTORS Byron B. Minnium and Leland Berry, Erie, Pa.,assignors to Erie Resistor Corporation, Erie, Pa., a corporation ofPennsylvania Application December 14, 1934, Serial No. 757,581

, 11 Claims.

This invention is directed to a method of making composition resistorelements whereby to produce the maximum percentage of serviceable unitswithin the predetermined resistance limits.

Composition resistor elements, that is elements made of electricalconducting material with a binder or binder and filler of non-conductingmaterial, have been made up by baking all of the units of a given groupat approximately the same temperature and for approximately the samelength of time, After baking, the ends of the units were sprayed withmetal or otherwise treated to provide low resistance terminals. Theunits so produced were then sorted by test into groups of predeterminednominalresistance values.

Where the units were above the desired resistance value, they weresometimes resprayed or retreated at the terminals to bring themapproximately to the desired value. In some cases as heretoforepracticed, where-one group of units had been baked and found to have atoo low or toohigh resistance value, subsequent groups of the samecomposition, in an attempt to produce approximately the requiredresistance value, were baked for varying time periods, and failing toproduce the desired result by these methods, it was usually necessary tochange the composition and readjust it in an attempt to reach thedesired result.

In the conventional methods now employed, however, it has been foundpractically impossible to treat a group of resistor units to produceeven approximately the same resistance values throughout any appreciablepercentage of the group, since for some unknown or undeveloped reasonindividual resistors of apparently the same composition responddifferently to the same or similar treatments. This is well recognizedand attempts have been made to bring individual resistor units, afterconventional treatment, to approximately the same'resistance values, bysubjecting them to the influence of a high frequency electricaldischarge.

However, it is generally recognized that up to the present time allmethods employed have been uniformly unsuccessful in presenting from agroup of .units a high percentage of completed units of approximatelythe same resistance values. 50

with a view to enabling the production of .a relatively high percentageof resistor units of approximately the same resistance value from atreated group and contemplates combining withthe more or lessconventional baking step an automatic The present invention is designedparticularly there is a well understood minimum baking period belowwhich the units will not present these de-' sired characteristics and amaximum b'akingperiod beyond which the desirable characteristicsreferred to would not be present. Thus, under all conditions theparticular group of resistor units must be baked or heat treated atleast for the minimum period and cannot properly be baked or heattreated beyond the maximum period. 1

As the resistor units must thus be baked or heat treated for at leastthe minimum period, it follows that the treatment, considered solely forthe purpose of producing a desired resistance value,-must be such as tocause the units to approach or reach this value between the minimum andmaximum baking periods. That is to say, the units in any event must bebaked for at least the minimum period and ordinarily the bakingtreatment is such that at this period at least the majority of the'unitswill not have reached the desired resistance value.

, As the effect of the baking is to gradually reduce the resistancevalue of the unit and, as previously stated, the resistor units ofanyone group will not all respond with like effect in resistance valuesfor the same baking period, it naturally follows that during theinterval between the minimum and maximum baking periodssome of theresistor units will reach the desired resistance value while others willstill be above that value. If the resistor units which reachapproximately the desired resistance value at a point intermediate theminimum and maximum baking limits are continued to be subjected to thebaking influence, their resistance values willbe lowered at the maximumbaking period beyond that desired in the particular unit.

Therefore, if the resistor units, after being subjected to the minimumbaking period, are continuously thereafter, during the interval betweenthe minimum baking period and maximum baking period, subjected tosuccessive tests for resistance values and if the testing means isarranged so that the resistor units subjected to ed by experience thatsuch tests are, when reaching the desired resistance value, ejectedbeyond the baking influence, it will be apparent that those resistorunits which, if permitted to be subjected to the maximum baking periodwould be of a too low resistance value, are, by the process of sortingand selection, delivered with desired characteristics, both ofresistance value and the other electricalcharacteristics noted. Thus, acomparatively large proportion of resistor units, which would otherwisebe of a too low resistance value at the maximum baking period, areusefully recovered at the proper moment.

In order to provide for the ejection oi. the units beyond the bakinginfluence when they have reached a predetermined resistance value, it isnecessary to provide the units with low resistance ends so that theresistance may be utilized to accomplish their ejection at theparticular time,

and as the unit as initially constructed-is not of such physicalformation as to permit of the application of the low resistance ends,this result is gained by initially baking each unit to a degree toprovide them with the requisite mechanical strength so that they may behandled.

Following this preliminary baking, the unit ends are sprayed orotherwise treated in any conventional or desired manner to provide thelow requisite ends to enable the resistance value of the unit to bedetermined during the subsequent baking of it to the desired resistance.

The units so treated are placed in an oven of particular constructionand subjected to a baking temperature. In order to prevent the metallicor sprayed ends of the unit from oxidizing in the oven, this bakingstepmay be. carried out in a reducing or inert atmosphere as, forexample, in the presence of nitrogen, carbon dioxide orother suitablegas.

As previously stated, and in order to" produce the desired electricalcharacteristics in a particular group of resistor units, the minimum andmaximum baking periods can be readily determined and it is known thatfor the particular units being treated, they must be baked for at leastthe minimum time and should not properly be baked beyond the maximumtime. As the heat treatment tends to gradually reduce the resistancevalues of-the units, it has been demonstratwhen the units have beentreated for the minimum baking period, some of such units will even thenbe of a too low resistance value for the desired units and others,ordinarily in greater number than the first group, will not have reacheda sufllcientlylow resist ance value even when the maximum bakingperiodhas been concluded.

There is thus an interval between the mini-' mum and maximum bakingperiodsdurin'g which a number of resistor units will have reached thedesired resistance value and if these are recovered at that particularmo desired electrical characteristics and the requisite resistancevalue. If the units reaching the desired resistance value in theinterval between the minimum and' maximum baking periods were permittedto continue to the maximum period, their resistance value ,would beto'olow for the desired result.

In order that the matter may be better understood, it will be suggested,merely for the sake of illustration, that for a particular group orunits ment, such units'have the the minimum baking period will be 30minute: and the maximum baking period 35 minutes Thus, in theillustration noted, the units cannot be baked less than 30 minutes ormore than 35 minutes and still retain the electrical characteristics,other than resistance values, desired. The automatic and selectivesorting step commence: at the limit of the minimum baking period andextends to the maximum baking period, the resistor units during thisinterval being subjected successively and at short intervals of time totests for resistance values and each unit reaching the desiredresistance value is ejected beyond the baking influence. I

As a result of the method, the resistor units are selected and recoveredas they reach the desired resistance value. Those units which, at theend of the minimum baking period, are found by the test to be of thepredetermined resistance value or of a too low resistance value areejected from the baking influence and may, of course, be later used 'asresistor units of a resistance value lower than that required of theparticular group being treated. Those resistor units which reach themaximum baking period without selection and ejectment will, of course,be of a too high resistance value for the desired groupand such resistorunits of a too high resistance value may be subsequently treated byrespraying or retreating the terminal ends to bring them within thedesired resistance value of the particular. group.

The method outlined consists essentially of selecting and interruptingfurther heat treatment of those resistor units of a group which,following the minimum baking period, have reached the desired resistancevalue prior to the maximum baking period; and further, treating theresistor units of a too high resistance value following the maximumbaking period to further terminal treatment to bring the resistancevalue to approximately that desired.

The method described can be applied to any type of resistance unit whichis th be baked to the required resistance, whether such unit be of Isolid composition, film type, core type, or other detail. Furthermore,the methodis not limited to units having sprayed or coated lowresistance ends. The process is equally applicable where the unit ismolded with low resistance .ends 'or to that type of resistance in whichthe low re sistance ends may be provided in any way inherently'in-theunit composition.- Under those circumstances, the initial step describedin con-' nection with the above method, namely the first 5 baking topermit handling of the unit and the subsequent application of the lowresistance ends, is not necessary and the method otherwise. describedmay be carried out with the particular type of resistance unit provided.6 In carrying out the method and for a better understanding of the same,reference is hadto the accompanying drawing, in which:

ratusjor carrying out the method. As. the pres- 7 a depending lip II atits free end for a purpose As shown,

ent application is limited to the method and no claims made for theapparatus, applicants illustrate and describe such apparatus merely as asimple ineans for carrying out the method and are not to be limited inany way as to any -detail of such apparatus.

l illustrates an oven of any desired form in which an endless carrier 2,such as a belt or the like, is arranged for travel over a predeterminedpath, being preferably supported on rollers 3. The interior of the ovenis heated by burners 4 and the oven may be formed with an appropriateopening 5 by which the resistor units, either automatically or manually,may be supplied to the supports on the carrier.

Secured to the carrier at appropriate intervals are supporting bars 6carrying a rigid curved arm I and a second similarly curved arm 8pivotally secured at 9 to the bar 6 and influenced by a spring Illtoward the arm 1. The arm 8 has which will later appear.

The arms 1 and 8 are of a size and shape to receive and support aresistor unit l2 and while the arm 8 is approximately semi-circular inoutline, the circumferential length of the arm I is of less length inorder to permit convenient discharge of the unit at the desired time.The supports described are duplicated adjacent the respective side edgesof the carrier to thus support the resistor unit at each end and thearms 8' are connected by a strip 13 of insulating material so that sucharms move in unison. Of course, but one of the arms 8 is provided withthe lip H.

The carrier, which is driven by any appropriate .means (not shown), isso timed in movement that during its travel from the feeding end to thepoint marked A will constitute the minimum baking period for theparticular group of resistor units, that is the period for which theresistor units must be baked in order to present the desired electricalcharacteristics, other than resistance value, above referred to. Thetravel of the carrier from the point A to the point B, the

. latter constituting the maximum baking period,

is defined by the lower flight of the carrier and throughout this flightand immediately below the carrier are arranged rigid bars i l and I5arranged in spaced parallelism with their relatively inner surfacesspaced apart to accurately receive and permit contact of the ends of theresistor unit l2 therewith during the travel of the unit in the movementof the carrier. Each of the bars M and I5 is provided at short intervalsin their length with contact strips l6. These contact strips aredesigned to be engaged by and form electrical contact with the sprayedor otherwise treated ends I! of each resistor unit passing between thebars.

Arranged below and carried by one of the bars, as It, is a solenoid l8for each contact strip IS. The core IQ of the solenoid is desired to beprojected under energization' of the solenoid into the path of movementof the lip II of the resistor support, this core being returned tonormal position by conventional spring means (not shown) ondeenergization of the solenoid. A resistance the oven to a suitablereceptacle, so that each resistor unitreleased from its support will bedelivered beyond further baking influence.

No attempt is made to show any details of the resistance meter, as suchis entirely and purely conventional, although, of course, any preferredor desired type of control may be provided.

The initial sorting station, that is to say, the one immediatelyadjacent the point of minimum baking influence on the resistor units,operates exactly as do the remaining sorting stations, and it will befound that at this station all resistor units which are of the desiredresistance or of too low resistance value will be ejected. The remainingunits travel with the carrier and are progressively brought into contactwith the successive sorting stations. If at a particular. station aresistor unit has a too high resistance value, the solenoid will not beaffected and such particular unit will move to the next station. If atthis station, this resistor unit is found of the proper resistance valueor slightly below such resistance value, it will be immediately ejected.

Thus, each resistor unit moving through the final baking period will besubjected to a test for resistance value and if the resistance value atthis test is of the proper value or slightly below such value, the unitwill be ejected. Therefore, during this final baking period, allresistor units which have reached the desired resistance value will beejected beyond further baking influence and thus usefully recovered andprevented from being brought to a too low resistance value by heattreatment to a maximum period which would otherwise necessarily occur;By the method and apparatus described, each resistor unit is testedsuccessively during its travel through the particular baking period inorder that, through the relatively numerous tests, the resistor unitsare re- "jected when they reach the desired resistance value.

The units which, at the limit of the minimum baking period, areautomatically ejected as being of a too low resistance value,may,ofcourse,belater used as units where a lower resistance value is required.Those units which reach the maximum baking period without ejectment, maybe subsequently treated in a conventional manner to bring at least partof them to the required value as is now practiced. By the methoddescribed, a considerable proportion of resistor units passingthrough-the oven are selected and elected at the desired resistancevalue and a very considerable proportion of those which pass through themaximum baking period of a too high resistance value may also be broughtapproximately to the desired value by conventional treatment.

What is claimed to be new is:

1. A step in the method of developing resistor units in a treatmentinvolving baking of the units to gradually change the resistance valueand develop electrical characteristics other than resistance value,consisting in subjecting the units successively to tests for resistancevalue during the baking treatment and ejecting from the treatment zonesuch resistor units as have reached a predetermined resistance value.

2. A methodw for treating resistor units to develop resistance value andelectrical characterwhile being subjected to a baking operation, testingeach unit at a predetermined point in such path, rejectingat such pointall resistor units having va resistance value equal to or less than thedesired value, and thereafter and during the baking operation subjectingthe remaining resistor units to successive tests for resistance valueand ejecting the units at \any one test which have developed the desiredresistance value.

3. A method of treating resistor units to develop resistance values andload and humidity characteristics, consisting in causing the units totravel through a predetermined path while be ing subjected to a bakingoperation, with such path including a minimum baking treatment point atwhich the resistor units have substantially developed the load andhumidity characteristics and a maximum point beyond which further bakingtreatment would jeopardize the load and humidity characteristics, andsubjecting the units between the minimum and maximum points of baking toa selective sorting and ejectment beyond the baking influence, with suchselective sorting and ejectment determined solely by a substantiallypredetermined resistance value of the unit.

4. A method as defined in claim 3, including the additional step ofselecting and ejecting at the minimum point of baking all units having aresistance value equal to or less than the predetermined value.

5. A method of treating resistor units to develop in successive units asubstantially predetermined resistance value, consisting in causing theunits to travel through a path while being suhacted to a bakingoperation, subjecting the units at predetermined points in such paths oftravel to a test for resistance value, and ejecting those units at eachperiod of test which have substantially reached a predeterminedresistance value.

6. A method of treating resistor units for developing resistance values,consisting in causing the units to travel through a path while beingsubjected to a baking operation, testing the units at predeterminedpoints in said path for resistance values, and interrupting at eachpoint of test any further heat influence on those units which havereached a substantially predetermined resistance value.

7. A method of treating resistor units to develop resistance values,consisting in feeding the units progressively through a heated zone,test- ,ing each unit for resistance value at progressive points -in.suchheat treatment, and bodily removing those units beyond further heatinfluence at each testing point which have reached a substantiallypredetermined resistance value.

8. A method of treating resistor units to develop resistance values,consisting in causing the units to travel progressively through a heatedzone, and periodically testing the units and removing from such heatedzone each unit which tests a predetermined resistance value.

9. A method of treating resistor units to develop resistance values,consisting in causing the units to travel in a predetermined path in thepresence of a baking temperature, testing the units in succession duringtheir travel through a final portion of said path, and ejecting thoseunits beyond the influence of baking operation which.

the resistance value of each unit at a predetermined point in suchtreatment and ejecting beyond treatment influence at such point theunits which reach a predetermined resistance value 'while continuing thetreatment of the remaining units.

